In general, the physical property of liquid crystal varies according to the state of the molecular alignment. As a result, response characteristic of liquid crystal with respect to external factors such as an electric field also vary considerably. Thus, it becomes an important technology to control the orientation of the liquid crystal uniformly in manufacturing liquid crystal devices. Many studies have been vigorously conducted to this end.
The uniform alignment of the molecules of the liquid crystals is difficult to obtain by merely injecting the liquid crystal between upper and lower substrates. Therefore, for uniform orientation, an orientation film is generally provided between the substrates for uniform orientation.
As the orientation method for liquid crystal, the molecular alignment is controlled by gradient vapor depositing inorganic materials (mainly, silicone oxide) without using rubbing treatment. The method using inorganic material, however, is being considered only in laboratory scale because large scale production is difficult due to the spatial non-uniformity and the low productivity thereof is not suitable for mass production. Therefore, organic orientation films obtained by coating an organic polymer, followed by rubbing with a piece of cloth are generally used. Among organic polymers, polyimides have been mainly used in consideration of the requirements for orientation films, such as efficiency in mass production, orientation efficiency for liquid crystal molecules and resistance to unfavorable environment.
However, the typical polyimide orientation materials have several disadvantages.
First, since highly pure monomer and solvent are necessary to synthesize polyamic acids (PA) which are precursors of polyimides, synthesis is difficult and costly.
Second, the solvent N-methyl-2-pyrrolidone (NMP) is highly hygroscopic and the above polyamic acids are degraded by water. Therefore, when PA is used or stored in an open system for a long period, its molecular weight decreases, thereby resulting in change in their physical properties.
Third, although inferior coating characteristics due to high surface tension of NMP are improved using a solvent having a low surface tension, i.e., butylcellosolve, it is still difficult to obtain a uniformly thin film of 600 .ANG. or less.
Fourth, in the case of polyamic acids, siloxane groups are incorporated in a polymer frame or a silane coupling agent for improving the adhesiveness to the substrate and a system added with a metal complex is used for regulating the contacting angle between liquid crystal and orientation film. Thus, uniform orientation control is hard to accomplish. Also, it is difficult to evaluate the affect of the interaction between molecular structures of liquid crystal and orientation agent on the molecular alignment, thereby making it difficult to select and design the liquid crystal and orientation agent efficiently.
Fifth, orientation characteristics of conventional orientation materials are liable to be changed by changes in device manufacturing conditions such as curing temperature or orientation conditions.
Sixth, specifically in the case of surface stabilized ferroelectric liquid crystal devices, since liquid crystal materials having a chiral smectic phase C (SmC) are used, if liquid crystal is injected in an isotropic phase and then the temperature is lowered, the liquid crystal becomes a smectic phase A having a layer structure perpendicular to the rubbing direction via a chiral nematic phase N and is again changed into the chiral smectic phase C so that the molecules within the layer are tilted at a specific angle with respect to the rubbing direction. At this time, as the gap between the smectic layers becomes reduced, bends in the smectic layers occur in order to compensate for the change of volume. This bent layer structure is called a chevron. Domains having different liquid crystal orientation are formed according to the directions of the bends. The non-uniform orientation is achieved where "zigzag," "hair-pin" or "mountain" damage is present on the boundary surface. As the result, the contrast ratio is lowered and a device of inferior bistability is obtained.
In order to solve such problems, improved orientation materials ('92 Japan Display, p579; and Liquid Crystal, Vol. 13, 1993), improved orientation treatment (SID '93 Digest, p364), improved liquid crystal materials ('92 Japan Display, p575), or orientation stabilization by an electric field (Japanese Journal of Applied Physics, Vol. 28, L119, 1989; and SID '91 Digest, p400) have been applied.
Also, in order to improve the bistability, which is one of the most important characteristics of a ferroelectric liquid crystal device, there have been known methods for using a conductive orientation film ('91 SID Digest), the orientation film to which a conductive complex is added, an LB PI orientation film (SID Proceedings, Vol. 30/4, 1989), and the liquid crystal to which a conductive complex is added. However, these methods have various drawbacks in terms of shorts, orientation efficiency or mass production.